1. Field of the Invention
The present invention relates to a bipolar device having a lateral collector structure on a buried oxide layer and a method for fabricating the same, and more particularly to a bipolar device having a high breakdown voltage for high power and operating at a high speed, by isolating a horizontal collector from a substrate through a buried oxide film and horizontally connecting a buried collector to a collector.
2. Description of the Prior Art
In general, to obtain a high output characteristic in a bipolar device, the collector breakdown voltage must be increased, by increasing the thickness of the collector or decreasing the impurity concentration of the collector. However, in this case, because the resistance of the collector increases, it is natural that the operating speed should be low.
FIG. 1 is a cross-sectional view illustrating a collector in a bipolar device, which is formed by a nonselective ion-implanting process.
As shown in FIG. 1, after forming a buried collector 2 by implanting ions into a silicon substrate 1, a collector film 3 grows on the resulting structure. After forming a field oxide layer 4, a collector sinker 5 is formed so as to extend the region of the buried collector 2 up to the surface, by using the ion implantation process.
At this time, if the thickness of the collector film 3 increases by H1, the resistance of the collector film 3 itself may increase. Furthermore, when impurities are implanted into the collector film 3, in the case where the maximum projection depth of the implanted ions is shallower than the thickness of the collector film 3, the resistance of the collector film 3 may increase more and more.
FIG. 2 is a cross-sectional view illustrating a collector in a conventional bipolar device, which is formed by the selective epitaxial growth process.
As shown in FIG. 2, after forming a buried collector 12, an insulating film 13 is formed on the resulting structure. When the thickness of a collector film is thinner than the diffusion depth of ions for forming a collector sinker, the insulating film 13 is etched using a mask to define an active region and a collector sinker region, and then a collector film 14 and a collector sinker 15 selectively grow. To make the impurity concentration of the collector sinker 15 high, impurities are implanted into only the collector sinker region.
When the thickness of the collector film 14 is thicker than the diffusion depth of the impurity ions for forming the collector sinker 15, after the collector region is opened and the in-situ doped collector film 14 selectively grows and an upper portion thereof is oxidized, the collector sinker region is opened and the in-situ doped collector sinker 15 selectively grows.
Accordingly, in the case where the thickness of the collector film 14 is two thick, the selective growth process must be carried out two times. In addition, since the rate of the selective growth is slower than that of the nonselective growth, it will take a lot of time to grow the collector film 14 and the collector sinker 15 by H2. Furthermore, the resistance of the collector may increase due to the thickness of H2.
As stated above, in the case where the thickness of the collector becomes thick, the conventional technique as shown in FIG. 1 must be carried out in the in-situ doping method since it is not possible to implant dopants into not only the collector sinker but also the collector film. Accordingly, if the field oxide film is formed by applying the thermal oxidation process, such as the LOCOS (Local Oxidation of Silicon) method, to the collector film in such a manner that a part of the collector film remains unoxidized, the parasite capacitance introduced in between the metal wiring formed on the field oxide film and the unoxidized collector film may increase. Thereby, the speed of the integrated circuits may become slow and a delay of signal transmission and signal attenuation may appear in the device.
In the isolation process, the field oxidation with the field stop ion implantation or the trench can be carried out. However, the thicker the thin collector film is, the deeper the trench is. Accordingly, since the width of the trench must be wide, it is difficult to decrease the area of the device. Also, the parasite capacitance between the buried collector film and the substrate further reduces the speed of the device.